1. Field of the Invention
The present invention relates to an image recording apparatus, and more particularly, to an image recording apparatus for recording images by discharging ink from a plurality of nozzles toward a recording medium, in which color mixing of inks is prevented.
2. Description of the Related Art
Conventionally, an image recording apparatus (inkjet printer) is known, which comprises an inkjet head (ink discharge head) having an arrangement of a plurality of nozzles and which records images on a recording medium by discharging ink from the nozzles while causing the inkjet head and the recording medium to move relatively to each other.
Various methods are known conventionally as ink discharge methods for an inkjet recording apparatus of this kind. For example, one known method is a piezoelectric method, where the volume of a pressure chamber (ink chamber) is changed by causing a vibration plate forming a portion of the pressure chamber to deform due to deformation of a piezoelectric element (piezoelectric ceramics), ink being introduced into the pressure chamber from an ink supply passage when the volume is increased, and the ink inside the pressure chamber being discharged as a droplet from the nozzle when the volume of the pressure chamber is reduced. Another known method is a thermal inkjet method where ink is heated to generate a bubble in the ink, and ink is then discharged by means of the expansive energy created as the bubble grows.
In an image recording apparatus having an ink discharge head such as an inkjet recording apparatus, ink is supplied to the ink discharge head from an ink tank storing ink, via ink supply channels, and ink is discharged by one of the various methods described above. However, in cases where a color image is recorded by using a plurality of inks of different colors, then if an ink of one color is recorded in an overlapping fashion onto ink of a different color recorded previously, before that ink has fixed and hardened, then color mixing occurs and image quality declines.
Therefore, conventionally, various methods have been proposed in order to prevent color mixing and thus improve image quality, when inks of different colors are recorded in an overlapping fashion.
In one known example, ink dots of respective colors ejected from respective color recording heads are recorded during one rotation of a rotating body supporting the recording medium, in a thinned-out fashion whereby the dots are separated by at least one dot space in the sub-scanning direction, which coincides with the direction of rotation of the recording medium (see, for example, Japanese Patent Application Publication No. 2000-71481). Therefore, mixing and spreading of adjacent ink dots is prevented, while achieving high-quality image recording.
In a further example, an inkjet recording apparatus comprises a device for estimating the state of drying of the recorded ink, and the recording intervals between one recording action and the next recording action are altered by changing the conveyance velocity of the recording medium and changing the interval between the recording heads, in accordance with the estimation results. In this way, it is possible to prevent recording irregularities in the regions where different inks are superimposed, while maintaining the through-put of the recording apparatus, and therefore, image quality is improved (see, for example, Japanese Patent Application Publication No. 03-247450).
Yet a further example describes a color inkjet recording method in which, in the same printing region, printing by at least one head of a plurality of heads is carried out after leaving a time period sufficiently longer than the printing delay time of the adjacent head, after the time of printing by another head. In this way, color prints which do not contain color bleeding can be obtained (see, for example, Japanese Patent Application Publication No. 04-173250).
However, in Japanese Patent Application Publication No. 2000-71481, for example, the dots are recorded in a thinned-out fashion during one rotation of the rotating body holding the recording medium, and therefore it is necessary to carry out a plurality of rotations in order to record one full image. Therefore, productivity is low.
Furthermore, Japanese Patent Application Publication No. 03-247450 comprises a device for estimating the state of drying and a device for adjusting the dot recording interval, in such a manner that the interval at which subsequent dots are recorded is adjusted in accordance with the state of drying. However, no device for fixing or curing the ink that has been deposited on the recording medium is provided, and the ink is simply left to dry naturally. Therefore, it is not possible always to prevent color mixing in a reliable fashion.
Moreover, Japanese Patent Application Publication No. 04-173250 simply sets a time period greater than the printing delay between adjacent dots, but it does not comprise a device for adjusting fixing or curing, and therefore has the same problems as Japanese Patent Application Publication No. 03-247450.